The present invention relates to a vehicle-mounted navigation system, and more particularly to a vehicle-direction measuring apparatus using a terrestrial magnetism sensor.
In a terrestrial magnetism sensor, a pair of magnetism detecting elements are disposed in the same plane and have a phase angle of 90.degree. with respect to each other. One of them is adapted to detect a terrestrial magnetism component in the X-direction (easterly direction), while the other is adapted to detect a terrestrial magnetism component in the Y-direction (northerly direction). If this terrestrial magnetism sensor is placed in a horizontal direction and is made to undergo one complete revolution, then the path of a circle, i.e., a magnetic circle, is depicted on X-Y coordinates by means of output values from both the X- and Y-direction magnetism detecting elements. Accordingly, if it is assumed that a central point of the path of the circle is (x0, y0), and that the X-Y coordinate data, which are output values of the terrestrial magnetism sensor, are (x, y), the direction .theta. can be expressed as EQU .theta.=tan.sup.-1 (y-y0)/(x-x0) (1)
However, in a case where the terrestrial magnetism sensor is mounted in a vehicle, in addition to the magnetization of a steel plate of the body, the length of the vehicle body exerts an influence on the direction .theta.. Since the body of an automobile has a rectangular configuration having a long longitudinal side, the perpendicular projection density of a magnetizable material such as a steel plate differs between the longitudinal direction and the transverse direction of the body. Therefore, even if the body is magnetized uniformly, the detection sensitivity of the X-and Y-direction magnetism detecting elements that are perpendicular to each other still differs.
Accordingly, the path of output values of the terrestrial magnetism sensor becomes elliptical when the vehicle with the terrestrial magnetism sensor mounted thereon makes one rotation rather than a circle, as shown in FIG. 5. If an ellipticity k (=Rx/Ry) is used to correct the output values (x, y) obtained by the terrestrial magnetism sensor, the output values become (x, ky-ky0+y0), so that the elliptical path is corrected to a round path. Hence, the direction .theta.M of the vehicle obtained by the terrestrial magnetism sensor can be expressed as EQU .theta.M=tan.sup.-1 {k(y-y0)}/(x-x0) (2)
An angular difference between north on the map (true north) and north obtained from terrestrial magnetism (magnetic north) is called declination. This declination occurs due to the fact that the pole of the earth and the magnetic pole differ. For instance, the declination of the Kanto district (including Tokyo) and its vicinity is -6.degree. (indicating that the magnetic north is offset by 6.degree. to the west from true north), while the declination of San Francisco and its vicinity is +15.degree. (indicating that the magnetic north is offset by 15.degree. to the east from true north). Accordingly, in the vehicle-mounted navigation system using the terrestrial magnetism sensor for measuring terrestrial magnetism, it is necessary to correct declination in order to determine an accurate direction. In addition, in the navigation system, a mounting error unavoidably occurs when the terrestrial magnetism sensor is mounted in a vehicle. A numerical value for correcting declination by taking into account this error as well is a declination correction value .theta.0. If the direction .theta.M of the vehicle is corrected by the declination correction value .theta.0, it becomes EQU .theta.M=tan.sup.-1 {k(y-y0)}/(x-x0)-.theta.0 (3)
With the conventional vehicle-mounted navigation system, however, at the time of setting the declination correction value .theta.0 the difference between the direction of terrestrial magnetism displayed on the screen and the direction of the road estimated at the present position is manually inputted by the user through a key operation or the like. Therefore, it is difficult to input a small value of several degrees as an appropriate value, resulting in a lowering of the accuracy with which the direction on the map is detected.